In order to realize high speed voice and data communication, optical transmission pathways have been designed and built that encode data in a modulated light signal and transmit the light signal with an optical waveguide, such as a glass or plastic fiber. The data is encoded and transmitted by a transceiver optical sub-assembly (TOSA) operating with an encoder, and received and decoded by a receiver optical sub-assembly (ROSA) operating with a decoder. Typically included in the TOSA is a diode laser, lens, and a circuit for driving the laser. Typically included in the ROSA is a lens, photodetector and amplifier for amplifying the photocurrent signal produced by the photodetector.
The form factor for a ROSA is generally a TO-46 can package, which is a 4-pin output package designed to mate with a fiber optic cable and corresponding LC connector. The four pins are usually dedicated to the positive polarity data, the inverse polarity data, the supply voltage, and ground.
The positive polarity data and inverse polarity data pins can be AC coupled to a data detector, which measures the optical modulated amplitude (OMA). The optical modulated amplitude, along with the extinction coefficient and the average power, define the attributes of the laser beam which comprises the data signal. The amplitude of the signal modulation may be measured by a radio frequency (rf) amplifier, and correlated to a root-mean-square (rms) value for the amplitude of the optical (AC) modulation. Such rf measurements can be sufficiently accurate indicators of the average detected optical power for relatively high power signals, however their accuracy is poor for lower input power levels. This is because the OMA measurement is a non-linear method, using an rf power amplifier which amplifies the noise power as well as the signal power. When the power level is low, it is difficult to separate the noise power from the signal power. Therefore it is difficult to detect a fault situation, such as a broken or intermittent link occurring on an optical transmission line, using optical modulation amplitude alone, because the noise contributes substantially to the overall signal level. Therefore techniques which measure the (AC) modulation amplitude are relatively poor indicators of changes in the attenuation of the data transmission channel.
Therefore, it is desirable to measure a DC average power level rather than the modulated amplitude (AC component) of the optical signal, and to output that signal to diagnostic circuitry.